The prior art is replete with various types of paging systems and radio-operated systems by which a message can be left for an individual who is not able to be contacted directly for one reason or another. For example, many paging systems operate with a large complex central processing facility in which messages are queued and transmitted, normally in digital form, to subscribers, along with the subscriber's address code. The subscriber to the service carries a paging unit which is preprogrammed to activate upon receiving a message which is preceded by the address code for that paging unit. The pager then normally emits an audible sound to alert the subscriber that a message is being received and recorded for him. The message is placed in the pager memory and the message is retrieved in the form of a written message, normally on an LED or LCD display screen. Although such systems are efficient and require very little air time in order to send the digital message, the messages transmitted are necessarily of limited duration and are normally of the type which require the subscriber to go to the nearest telephone and call the message originator. In addition, unless the transmitted messages are strictly numeric, i.e., telephone numbers and the like, alphanumeric messages require a special terminal in order to input the alphanumeric message to be transmitted, and paging systems of this type require expensive computerized central message facilities.
Other paging systems are available which utilize a transmitter which transmits in analog form an audio message preceded by an address code which is received by a preprogrammed receiver. The message is played immediately upon receipt, and, in some units, the message can be recorded on a tape cassette for replay. Pagers of this type are normally relatively bulky and require substantially high power requirements to drive the mechanical portions of the tape recorder.
In the area of telephone communications, typically available answering machines have one or more tape cassettes for playing a message to the caller to indicate that the called party is not available to answer the phone and to record a message for later playback. Although answering machines are readily available for single-line use and their price is becoming more and more reasonable, such devices are normally not available as part of the telephone circuitry itself. Most of the existing answering machines are bulky and require a substantial amount of desktop space. Furthermore, answering machines are not readily available for multi-line business phones and com-line, cellular or mobile recording.
Also in the area of telephone communications, the advent of cellular telephones has revolutionized mobile communications and has created a need for enhanced capabilities. Cellular telephones utilize sophisticated cellular technology and are faced with growing demand. Ongoing research in this area concentrates on providing additional conveniences and reducing costs. For example, presently, when a particular cellular user is away from his or her unit, a general recording from the central cellular station or site informs the caller that the particular user cannot be reached at the time. Thus, the caller has to take the initiative of calling again, which can result in several unavailing and frustrating attempts.
Most existing cellular telephones incorporate the capability of indicating to users if they received any calls during their absence by some sort of an indicator. This feature is sometimes useful and reassuring, especially when the user is expecting an important telephone call from a particular person while away from the unit. However, this feature is inadequate and does not solve the problem since it does not identify the caller, record the number at which the caller may be reached, nor indicate the total number of callers and their messages. Although answering machines for this purpose are available for standard telephones, such capabilities are not available with cellular telephones. Reducing costs is also another high priority since cellular telephones are relatively expensive because calls are typically billed on a per-minute basis.
Yet another form of message service is the so-called voice storage retrieval system (VMS) in which a voice message can be left at a central message storage facility, and the subscriber, by use of a specific code, can access the memory at the central computer to retrieve the message. These systems are expensive to operate in view of the necessity of powerful computers at the central system facility to process and store the messages and, in addition, can be inconvenient to use since the subscriber must be at a telephone in order to receive the message. In addition, messages may not be timely received because the addressee inadvertently fails to check for messages.
In the area of two-way radio communication, such as in the case of police and fire communications, emergency communications and the like where the addressee may be away from the mobile unit from time to time, many systems employ the use of hand-held receivers, i.e., walkie-talkies, which may be patched into the mobile receiver for the receipt of incoming messages while the operator is away from the unit. Such devices are expensive and, in many cases, would be totally unnecessary if a reliable, inexpensive message storage system were available at the mobile unit. Some systems are available which are similar to the telephone answering machines for transmitting a prerecorded message and for recording incoming messages when the operator is not at the mobile unit. These systems have been found to be bulky, unreliable and inflexible in connection with radio communications.
A more sophisticated system has been promulgated in U.S. Pat. Nos. 4,468,813 to Burke, et al. and U.S. Pat. No. 4,495,647 to Burke, et al. This system requires a base unit which sends a command program packet in digital form to the mobile unit which is programmed to respond to the command program for receiving the message in analog form. The command packet includes numerous command codes, transmission of which require significant air time. Responsive to the command program, the mobile unit converts the message to digital form for storage and responsive to a termination command sent in digital form by the base unit, the mobile unit recording system is deactivated. The operator at the mobile unit can then replay the digital message in analog form. The system as disclosed in the aforementioned U.S. patents requires a sophisticated encoding system at the base transmitter which is capable of generating a command program packet and the termination code signal. Furthermore, the base transmitter must be capable of transmitting the command packet in the form described in the aforementioned patents. The mobile unit must be capable of receiving, decoding and transmitting the command program packet back to the base unit. The mobile unit utilized in such systems requires two separate power supplies which would render the device unsuitable for portable hand-held receivers, such as pagers and the like.
In the communications field, particularly in radio communications, transmission time is desirably maintained at a minimum. In radio communication, the available channels are crowded and there is great competition for air time. For this reason most paging systems involve the transmission of messages in digital format because the digital format requires less time to transmit. The messages received, however, are limited to short written messages which are displayed on a small LED or LCD display screen and the messages are normally limited to the type that require the subscriber to go to a telephone and call the message originator. In addition to the limited message capability of such systems, the transmitting components are expensive, normally requiring centralized computer message facilities to transmit the digital data and to store the analog messages for the subscribers.
Accordingly, it would be highly desirable to provide a system for transmitting analog messages directly to a remote unit in which the analog messages are transmitted at a high rate of speed to conserve air time and are received and recorded at the remote unit for playback at a slower rate of speed to return the message to its audible condition. As a corollary, it would also be desirable to transmit certain analog data at a slow rate of speed and to play the message back at a higher rate of speed, again to return it to its audible format. Such a transmission procedure is beneficially carried out in the transmission of music and other high fidelity analog data over telephone lines which normally adversely affect the fidelity of the data being transmitted. In such a case it would be desirable to transmit the data at a slow rate of speed thereby to retain the fidelity and to play it back at its normal rate of speed without loss of fidelity.